Epoxy resin compositions containing epoxy resins and curing agents for the epoxy resins as essential components exhibit good heat resistance and insulating properties when cured and thus are widely used in electronic part usages such as semiconductors and multilayer printed boards.
Among the electronic part usages, the technical field of the multilayer printed board insulating materials has recently seen an increase in the speed of signals and frequencies in various electronic appliances. However, with the increase in speed of signals and frequencies, it has become difficult to obtain low loss tangent while maintaining a sufficiently low dielectric constant.
Accordingly, it is desirable to provide a thermosetting resin composition that can be cured into a cured product that exhibits a sufficiently low loss tangent while maintaining a sufficiently low dielectric constant for signals with higher speed and higher frequencies. Regarding the material that can achieve low dielectric constant and low loss tangent, a technique of using, as an epoxy resin curing agent, an active ester compound obtained by aryl-esterification of phenolic hydroxyl groups in a phenol novolac resin is known in the art (refer to PTL 1 below).
Under trends toward higher frequencies and size-reduction of electronic parts, multilayer printed board insulating materials are also required to exhibit a significantly high level of heat resistance. However, the aforementioned active ester compound obtained by aryl-esterification of phenolic hydroxyl groups in a phenol novolac resin decreases the crosslinking density of the cured product due to introduction of aryl ester structures and the cured product does not have sufficient heat resistance. As such, it has been difficult to achieve both low dielectric constant and low loss tangent.
Materials used in this field need to address environmental issues such as dioxin-related issues and there is increasing demand for halogen-free flame-retarding systems in which a flame-retarding effect is imparted to the resins without using an additive-type halogen-based flame retardant. However, the aforementioned active ester compound obtained by aryl-esterification of phenolic hydroxyl groups in a phenol novolac resin contains many easily combustible pendant-type aromatic hydrocarbon groups in its molecular structure despite the improved dielectric properties, and thus the cured product of this compound has low flame retardancy. The halogen-free flame-retarding system cannot be established with this compound.